For example, one might say that...

$x -= 1

..is a statement. "Decrease the value of x by one."

Compare with:

while ($x -= 1) { ... }

...where the same statement is being used as an expression.

You should mentally parse the words statement and expression as just "piece of code".

The EXPR context is another story however, and a has a very specific meaning to the Perl parser. That is an aside however. When people say expression they rarely mean precisely an EXPR context.